Not really. I'm only willing to do this because I firmly believe that BTC will increase in value by a lot more than 20%
I don't follow. Why don't you charge the full £100 and then using it to buy £100 of BTC, instead of accepting £80 worth of BTC? Offering a discount for BTC means you end up with less BTC.
As an aside, your freedom to offer a discount for BTC is why Gresham's Law doesn't apply to bitcoin. There may come a time when such discounts are used to encourage bitcoin hoarders to spend. That will lead to more deflation, while increasing the velocity of money.